Soft start control method for a motor-driven actuator

Abstract

A method of operation for a motor-driven worm gear actuator coupled to a load device energizes the motor in a series of pulses during an initial period of motor operation when reversal of the motor is commanded. The pulsed energization produces a slow initial axial movement of the motor armature without moving the load device, minimizing the contact force between the armature shaft and a mechanical end stop of the motor. Rotation of the motor is determined by identifying and counting motor current pulses due to commutation, and the pulse count is adjusted at each reversal of motor rotation to compensate for motor current pulses due to the pulsed energization.

What is claimed is:  
     
       1. A method of operation for an actuator that positions a load device in accordance with a position command, the actuator including a reversible electric motor having a rotary armature shaft and a gear arrangement including a worm gear disposed on and driven by the armature shaft for coupling the armature shaft to the load device, wherein the armature shaft shifts axially into engagement with armature end stops when a direction of armature rotation is reversed, the method comprising the steps of: 
       detecting a commanded reversal of armature shaft rotation;  
       when a commanded reversal of armature shaft rotation is detected, energizing said motor in a series of pulses sufficient to move said armature shaft axially into engagement with a respective armature end stop without moving said load device; and thereafter  
       fully energizing said motor to move said load device in accordance with the position command.  
     
     
       2. The method of operation of  claim 1 , including the step of: 
       detecting rotation of said armature shaft by identifying and counting motor current pulses such that the detected rotation is insensitive to motor current pulses caused by said series of pulses.  
     
     
       3. The method of operation of  claim 1 , including the steps of: 
       turning said motor on and off for predefined on-time and off-time intervals when the commanded reversal of armature shaft rotation is detected; and  
       fully energizing said motor after a predetermined number of said on-time and off-time intervals.  
     
     
       4. The method of operation of  claim 3 , including the steps of: 
       counting said motor current pulses to form a pulse count; and  
       reducing said pulse count by said predetermined number to compensate said pulse count for motor current pulses caused by turning said motor on and off.